To elucidate the genetic basis of olfaction, the mechanisms that maintain genetic diversity and the origin of vertebrate olfactory receptor gene family, we have tried to clone the olfactory receptor genes of Drosophila and medaka fish (Oryzias latipes).From medaka genomic DNA,four receptor genes, named mfOR1, mfOR2, mfOR3 and mfOR4, were successfully cloned using a set of degenerate oligonucleotides as PCR primers. All of their deduced amino acid sequences contained seven hydrophobic domains and several sequence motifs commonly seen in the olfactory receptors. The four medaka receptors exhibit from 19-91% sequence similarity to each other. At amino acid level, four receptor genes show high homologies with the olfactory receptor sequences from catfish and zebra fish. These results suggested that they encode novel members of olfactory receptor gene family.Linkage analysis have revealed that the four receptor genes are grouped into two clusters. The cluster of mfOR3 and mfOR4 linked in medaka linkage group XX which showed synteny with the mammals. Furthermore, phylogenetic analysis by NJ method revealed that medaka olfactory receptors branches well from the tetrapod vertebrate.